After coupling or uncoupling the semi-trailer, the landing gears are telescoped, i.e. they are extended or retracted, and support the standing semi-trailer. This extending and retracting of the landing gears is usually done by the driver using a hand crank that is mounted on one of the landing gears. To relieve the driver of this physically strenuous work, efforts are being made to apply the power required to retract and extend the landing gears by a motor.
Such an arrangement of a motor on a landing gear is disclosed in EP 0 179 632. The motor is mounted to one of the landing gears from the outside—in place of the hand crank—to a primary shaft of the landing gear gearbox. The essential drawback of the power-driven landing gear of the prior art is that the motor is mounted on the outside of the landing gear in an unprotected area and, in practice, is frequently damaged. A further drawback is that the motor must be designed for a low speed and a high speed of the landing gear, which entails substantial motor design complexity because it requires either a motor control or an additional motor speed.